U.S. Pat. Nos. 3,874,822 and 3,877,841 disclose one form of prior art solenoid-actuated fuel oil pump in which an electromagnetic plunger or armature surrounded by an electromagnetic coil is supported within a main pumping chamber by coil springs abutting opposite ends of the armature. Connected to the lower end of the armature is a pressure plunger or piston whose lower end telescopes into an intake pressure chamber. The latter is separated from the main pumping chamber by a check valve on the discharge side of the intake pressure chamber. Another check valve on the suction or inlet side of the intake pressure chamber keeps fluid from being pumped back through an inlet side of the intake pressure chamber. A half-wave rectified alternating current applied to the coil provides for intermittent energization of the coil because current flows only in one direction through the coil in a cycle. When current is flowing through the coil, the armature is driven upwardly to compress the upper one of the coil springs by the electromagnetic forces resulting from energization of the coil. Then, when current flow stops, the stored forces in the upper spring push the armature downwardly, its momentum propelling it past a neutral spring force position in the main pumping chamber thereby compressing the lower coil spring. As current is reapplied to the coils, the armature again is forced upwardly against the upper spring.
In the present solenoid pump as well as the prior art pump described above, the power stroke of the piston is with the upstroke of the armature so that, as the piston enters the main pumping chamber from the intake chamber, fuel oil is forced through a longitudinal passage in the armature and out of the main pumping chamber through a hole in a magnetic force adjusting rod or plug which also serves as the upper reaction member for the upper armature spring.